Wastewater denitrification protects water resources from nutrient enrichment and accelerated eutrophication. Problems associated with eutrophication include excessive algae growth, turbidity, foul taste and odors, accelerated sedimentation, pathogen growth and hypoxia. Such issues are exacerbated when the wastewater is discharged into a lake, reservoir, estuary or delta, but most pronounced when the discharge is a large portion of a stream or river flow. As a result, the regulation of nitrogenous wastes is bound to become more stringent in the near future. Reliable and cost-effective means to reduce nitrogen concentrations to increasingly lower levels are needed. In response, biological denitrification—bacterial conversion of oxidized nitrogen contaminants to harmless nitrogen gas—has received some attention in the art.
One such approach is described in U.S. Pat. No. 6,307,262, the entirety of which is incorporated herein by reference. A hollow-fiber membrane biofilm reactor (MBfR) introduces hydrogen gas as an electron donor to induce growth of hydrogen-oxidizing bacteria on the membrane's surface. Such bacteria, in turn, reduce oxidized nitrogen contaminants to nitrogen gas. Hydrogen is an efficient and cost-effective reagent that avoids the toxicity and material-handling problems associated with organic electron donors of the prior art. The MBfR alleviates many prior concerns associated with hydrogen, including low solubility and high flammability. Hydrogen diffuses from lumen of the fibers toward the aqueous medium, promoting biofilm growth. Use of a hydrophobic construction material permits the membrane pores to remain dry. No hydrogen bubbles are formed, and little or no hydrogen is carried out but through the treated water, minimizing additional oxygen demand.
Even so, the MBfR technology embodied in the '262 patent does not always provide a complete solution to wastewater treatment. For instance, the hollow fiber membranes are confined to a tubular configuration and require, by design, movement of the aqueous contaminants along the longitudinal fiber axes. One or more water pumps are needed for recirculation and continuous reaction. The tubular configuration does not lend itself to existing wastewater treatment basins, and the fiber density impedes movement of solids through and out of the reactor.
One of the emerging challenges for wastewater treatment is achieving very low effluent concentrations of total nitrogen (TN) and total phosphorus (TP). Increasingly severe problems with eutrophication and hypoxia in lakes, reservoirs, estuaries, and the near-shore ocean are forcing environmental regulators to impose more stringent effluent requirements on TN and TP. For example, an effluent standard for TN could be 1 mgN/L when the discharge is to a sensitive water body; it is possible that a receiving-water standard of 0.12 mgN/L could be applied if the wastewater were the dominant water input.
Existing wastewater-treatment technology is capable of taking effluent TN down to the range of 10-15 mg//L, but it is neither reliable nor cost-effective for achieving ≦1 mgN/L. A key for taking TN down to the 1-mg/L level is stable denitrification to drive NO3−—N to a few tenths of a mg/L. Stable nitrification can drive NH4+—N to a few tenths of a mg/L, and filtration can bring organic N to almost zero. If soluble organic nitrogen can be held to a few tenths of a mg/L, total N could be reduced to about 1 mg/L: e.g., 0.2 mg/L NH4+—N and 0.3 mg/L NO3−—N totaling 0.5 mg/L soluble organic N.
Pre-denitrification can utilize influent biological oxygen demand (BOD) to fuel denitrification, but realistic constraints on the mixed-liquor recycle rate limit it to about 75% N removal, which leaves about 10 mg/L TN in the effluent when the influent is 40 mgN/L. Furthermore, a high influent TKN:BOD ratio can foil the pre-denitrification strategy as a means for total N removal. Return of digester supernatants is a common situation leading to a high influent TKN:BOD ratio.
Tertiary denitrification using an organic electron donor, such as methanol or acetate, could, in principle, drive effluent NO3− to a few tenths of a mgN/L. However, the dosing of the organic donor cannot be controlled well enough to ensure full NO3− removal without massive donor overdosing that increases effluent BOD and wastes money. In addition, tertiary denitrification using an organic donor significantly increases excess sludge production and often involves special chemical handling. For example, methanol (CH3OH) is popular for its relatively low cost, but methanol is a dangerous chemical that is toxic to humans, is regulated, has very difficult handling properties, and is oxidized only by specialized methanotrophs.